Hmmm. That's a hard one. You want to be diplomatic about it as well.
Well, let me tell you a little story. There's this guy in my choir who's really nice, but he doesn't sing in tune most of the time. He tries, but he always says he's not very musically inclined. I've offered to help him work through the music, using the excuse that it'll help sharpen my sight-reading skills on the piano (and he's always trying to get me to play the piano for him!). So one time, he was over at my house, and he had missed the previous rehearsal, so I was playing some of the pieces and singing along to them. For some of them, in which the choir sang in unison, he sang along with me, and I helped him with some problematic spots. For those in which we broke off into parts, well, that was a different story. He sings tenor but I call him the reluctant tenor as he would much rather sing the melody. So he hemmed and hawed and at the end just ended up listening to some other pieces I was playing until it was time for him to go home.
Certainly offer the help. It'll be up to the other person whether or not s/he'll accept. But I'd phrase it in such a way so that it would be like you wanting to practise a section together so as not to seem threatening in any way.
Hope this helps!