How would you break it to someone that their DPS is awful?

[Gextiv]

I ask if they game on a PS3(i.e. consoles in generally). I then play it off as curiosity, pointing out that their character moved oddly.

Usually they get the message.

[Elim]

Be tactful. While he is in TS(or whatever chat program you use) start an in-depth conversation on DPS in general. At some point start to talk about whatever class he plays, make sure to get him involved with the subject. Tell him you want his opinion. Link a rotation you think is good for the class, let him say his opinion on it, counter whatever he says is better or whatever, etc. This way it's just a " casual conversation you struck up".

[Delorean]

Why do people advocate being anything less than perfectly clear about an issue that is bogging down the raid? Do you really want to play the game with someone who is going to flip shit about not making a worthwhile contribution and isn't willing to make an effort to?

Just catch them after raid and tell them about it and that you'd appreciate it if they tried working on it. Because to me it doesn't sound like it's highest on your list of priorities, but that it'd be nice if they could try to contribute just a little bit more.

[Zarzak]

Why do people advocate being anything less than perfectly clear about an issue that is bogging down the raid? Do you really want to play the game with someone who is going to flip shit about not making a worthwhile contribution and isn't willing to make an effort to?

Just catch them after raid and tell them about it and that you'd appreciate it if they tried working on it. Because to me it doesn't sound like it's highest on your list of priorities, but that it'd be nice if they could try to contribute just a little bit more.

I don't think anyone is advocating it. The OP specifically expressed her desire to keep the player in the group. So within those bounds people are providing the best possible solutions.

[Sagittarian]

I approach it like asking for help with the class myself.

"Hey X, I'm working on my [job] rotation. You like playing [job]. What do you do? Do you use this [link] or something else?"

[Souljacker]

I approach it like asking for help with the class myself.

"Hey X, I'm working on my [job] rotation. You like playing [job]. What do you do? Do you use this [link] or something else?"

That just gives the player completely the opposite impression from what the OP is trying to do. The player isn't successful at DPS, and frankly, it sounds like he might not be very successful at life, either if he can't take constructive criticism. I'm always for spelling out the truth in a tactful way. Express to him how valuable he is to the group, but be clear that his DPS skills are not up to the level his tanking skills are. If he really is so delicate that you can't improve him without him falling apart - he (and you) are going to have bigger problems down the line both in RL and in the guild.

[ninthfantasy]

I've had someone just /tell me "Hey I can give you some tips on xyz if you want " and I was like "Oh okay cool." Haha. Obviously it was there way of saying "Your DPS is lacking a tad." But it seemed like they genuinely wished to help rather than condemning me for not having optimum DPS, which made it sound a looot better and didn't make me feel so bad. Obviously it's really up to the person on how they take it, and it depends on your relationship to them, but if it's FC member/static member, I think just /tell them with a friendly message about rotations and tips or maybe just saying, "Found this cool guide for a rotation, thought you might be interested!" is a non-direct way of them getting to reconsider their dps/rotation, plus it's less confrontational and probably won't make them feel as bad.

[seraf]

I'm too nice to just tell someone that their DPS is low so I don't usually bring it up. I have offered rotation advice though if I notice they're being inefficient with it. Most people will say thanks but you still get the occasional "DON'T TELL ME WHAT TO DO" response.

[Novaura]

When I started getting to Level 50, tanks always gave me a hard time for not using all of my abilities. The main reason I didn't use them was because my skill setup was used for fighting enemies one on one in regular quests and guildleves. So I had no idea how to use my actual DPS skills effectively and people never told what I was doing wrong primarily because the people who harassed me the most were when I was in Brayflox's Longstop (Hard). But the damage was done and I became uncomfortable to ever go to Bray Hard again knowing who I would be dealing with would be very vicious.

Fortunately, when I learned how to use the DPS skills that were meant for use in dungeons and raids, the haters went away and I started to get a lot of commendations for my work. So give it time help those who don't know about how they can use their DPS skills to know how they can use them. Especially if you have done that DPS class before.

[Sagittarian]

That just gives the player completely the opposite impression from what the OP is trying to do.

It's just the opener so they don't start on the defensive. It would then be followed up with pointed questions like "Just flare not swiftcast+flare?", "Isn't flare less efficient for single targets?", or "Blizzard 1? Why not blizzard 3 to get a full stack of ice?". I've found it more effective with certain personality types.
And I'm assuming this a /tell conversation. If in group chat then I'd ask everyone with that job to chime in with what they think is best.

I've seen so many people use flare against a single target without even using swiftcast.... I sit there and watch their cast bar for 4 seconds in utter amazement.