Your buddy is probably being optimistic, and that optimism may not be grounded in how judges actually look at this situation.
Catching new felony charges while already serving time or on supervised release is about the worst signal you can send to a court. The existing sentence was already a message that the system sent once. New charges, while on paper, tell a judge that the message did not land. Courts do not respond well to that, and the idea that Class D felonies are not serious enough to matter is a gamble that does not usually pay off.
There is not enough detail here to say with certainty what happens next. The specific charges, the jurisdiction, the judge, his prior record, and the terms of his supervision all factor into the outcome. Some situations do resolve with release and extended supervision. But that is the less likely path, not the more likely one.
What typically happens is that the new charges trigger a violation of whatever supervision or remaining sentence he had, which gets stacked on top of whatever he is facing for the new felonies. Judges tend to view repeat offenders as people who have not absorbed the lesson yet, and they sentence accordingly.
He should not be making assumptions about his outcome right now. What he should be doing is getting the best legal representation he can find and letting an attorney assess the actual facts rather than talking himself into a result he wants to hear.