Speakers for small room

Lots of companies make slim on-wall speaker options. I haven’t heard many but would suggest choosing an option that extends below 80hz at a minimum because it will make integrating the subwoofer easier. Frequencies below 80hz will be less easily localized to the sub. Many of those super thin on-wall options would leave you with crossing over to the sub way too high. Here are two, but I haven’t heard them and I’m sure there are many others:

Minimum frequency on those is 110hz and 100hz, respectively. That means your subwoofer will need to be set high enough to avoid missing those 80-110hz frequencies (80hz being a more common low pass setting than what you’d need with these). Make sure the AVR can adjust accordingly and is set to do so. With the crossover that high, you’ll likely have sound that can be localized as coming from the subwoofer instead of just appearing to be from the main speakers. It’s suboptimal (pun intended), but if he likes them enough for other reasons might be a trade off he’s ok with. Personally I’d pick something that goes to 80hz or lower because that’s about the frequency where our ears/brains can’t localize the sound.

Those are in-wall rather than on-wall speakers. Just making sure he knows he would need to cut the drywall and mount those into it rather than just securing it to the face of the wall… I have in-wall speakers in my basement and had them in the living room of a previous house, so I’m not knocking them at all, but installation has a few more pitfalls to avoid. Basically you need to know where the studs are and if anything else is in the area between them for each place you want a speaker. Probably best to be at least capable of minor drywall repair in case he comes across something unexpected. If it’s a house he owns then the in-walls can be great, but wouldn’t choose that route if his place is a rental.