Photo collages/scrap books are almost always a welcome addition . .. I made one for my Mom that included my daughter from early years through high school graduation (which was her age at time of book) - I didn't live too close when she was growing up so mom never got too much visit/watch her grow time . . . anyway - she loved it.
as for your questions:
1. The life of dad and family/friends . . . good idea.
2. Include late wife AND new girlfriend . . . of course. I am of course assuming that girlfriend came after wife and not at the same time (I know you already said she is a recent addition to his life). They are both part of his life and who he was and now is . . . by all means include both . . maybe not on the same page, but in the same "book".
3. Format: If you have the time and enough "data" I would do a physical scrap book, leave or add a few blank pages with maybe a "post-it" that says "to be continued" or "we will contiune to update you as the story unfolds" or something like that. . . that way you can add pages as events/years/etc happen.
If time and or pictures don't allow, then maybe a family tree sort of collage to be framed. . . Mom & Dad at top of "page" with "groups" below showing "brothers, sisters, uncles, etc" and "girlfriend (unless they are really serious/getting hitched, then she can go at the top on the other side of him from Mom), friends, fishing buddies, etc".
Or even do a calendar with a differnt assortment of people each month (maybe based on special dates or birthdates)
The format really needs to depend on your time available to spend, his wall space/decorating scheme, and the number of pictures you have.
Good luck and remember, just make it mean something while you have fun with it.