playing through other members' games

I was playing through a 1500+ members games(10minutes) and noticed that he had missed a mate in one. Do you think he would be grateful to be informed or will I be inviting a stream of expletives?

Depends on how you do it and who you do it for.  If someone let me know something like that in a matter of fact way, I believe i would be grateful.

("I noticed Qh4#.")

Puzzled as to why someone had bothered to look over my games but grateful.

I think most serious players look over their games

I think they sold find it silly and maybe offensive as its pointing out obvious

you should tell him

he might give you $10 for the free lesson

If they are serious about improving i would think they would appreciate it.