Okay - I would say you're "on the money" - its either C or G. It's a bit ambiguous tbh. I would suggest it would be G however...
I base that idea on the fact it starts in G and G then gets mentioned again etc. To fit in with this 'best' guess... Heres the Key of G:
Although usually people stay well away from F#°
Or you could borrow some of these chords from the Parallel Minor Key of G minor:
(Again stay away from the diminished chord = A°)
This is likely what your Hubby has done just that and borrowed the F from the the G minor ke... if you look at the G Major then at the G minor Key he has replaced the F#° with the G minor Keys note of F. See what I'm saying?
Anyway to keep it simple I'll leave it there for him/you to explore, however below are some secondary dominant options which might help him too:
If you play the smaller chords first then go into the main chord above it it will sound nice.... i.e. play D7 then play G etc.
There are loads of things he could do really but I hope that is a helpful start...
P.S Here's a useful link to a tool which this site's creator has made for determining Keys: