Can you still impress colleges in your senior year?

Well, while taking AP classes in his senior year won't have the same advantage as they have had you, they don't hurt. If he applies early anywhere, colleges will get a list of classes that he's taking this year and will be able to see that he has decided to challenge himself rather than taking the easy way out, which does look favorable. That being said, taking a bunch of them suddenly in his senior year will not only look as if he's taking them specifically for the purpose of getting brownie points, but may also cause his GPA to tank (which is proving to be happening to a lot of people in my class who decided to take a bulk of AP courses in their senior year). Taking AP classes looks good, but taking a bunch of AP classes and doing terribly in them because you aren't accustomed to the level of work looks bad (especially once colleges do receive one's grades from that first semester of senior year).

That being said, I do reccomend your friend tries them but maybe only one course (two at the most if he really wants that challenge). It doesn't hurt to try and, especially if he gets an interview with an admission counselor where he can talk about how he decided to take the risk of the challenge, it does ultimately look good. Is he going to have the same chance as you? Probably not, but we can't be completely sure because college admissions are never a sure thing and he might win admissions over with a great essay or community service/extracurriculars or maybe his sparkling personality.

Yes