You need to post your question to a US immigration forum. I don't think you'll get the answers you're looking for here.
What I can tell you is the following...
If the boyfriend entered legally (i.e. as a visitor) and he and your daughter get married - then she will be able to petition him (essentially sponsor him) for a Green Card and his overstay will be foregiven. Unfortunately he has no grounds to do this alone. Your daugher must be part of the process. To qualify to sponsor him, your daughter must be making an annual income that is 125% the poverty income line. If she does not make enough money, you can co-sign the application.
The boyfriend should not leave the US until his status has been adjusted and he has his Green Card. I don't know how old he is now. But depending on how old he is, if he were to leave the US now, he could get slapped with a 10 year ban from the US for his overstay. So it's very important that he stay put until he's legal.
You'll need to join a US site for more details about how to kick start this process.
I have the link to a good US immigration forum at home. I will post is here tonight.