I don't have any personal experience with reunification therapy but my dd was reunited with her dad last fall after a 4 year absence in her life so I can share my experience with that.
My two biggest pieces of advice for your friend would be, #1 be more of a friend at first, then move slowly into being dad. My dd had a lot of moments where she would get peeved at her dad for trying to lay down the law. Ex. Eat everything on your plate. We've never been eat everything off your plate people, so she would say no, and then he would get upset. He would expect me to back him up, just because he is the dad and he said so. It put me in a weird place, when I'm trying to help them bond, and then I'm being put in the middle of such a trivial issue.
Dd was still trying to understand why she hadn't seen her dad in 4 years, and it was hard for her to take advice, or rules from him. Actually it still is.
#2 be consistent. About 2 months ago dd's dad stopped contacting her as much as he was. It has oddly not effected her as much as I thought it would, except that she is now ultra clingy with my Dad (her Pap). If her dad thought that he could ever slow down contact with dd, then I wish he had kept it slow from the beginning.
If I think of anything else I'll post it.