If it was a normal person, you would think they are just being helpful, but the fact when Dianne is around, she is prominently featured in his vlogs instead of his girlfriend.
But I think what makes his behaviour move from just awkward and socially inept to inappropriate is a clip in Dianne's vlog. When she is going to jump from the halfpipe to the trampoline, Alfie tries to give her a hand, but actually steps away from him. It looks like she falls in Alfies vlog, but in Dianne's you see he keeps trying to to "help" and when he goes to put his hand on her back, is when she goes over. Looks like Dianne chose to fall rather than have Alfie touch her.
Alfie seems harmless enough and Dianne is hot and has a fun personality. But, he really needs to stop crushing on his mates girl especially when he goes out with that mates sister.
Joe is the jealous type to the point, he very rarely hangs out with Dianne and any of the single Buttercreams together. So don't think he will be too happy with Alfie drooling over his girl.