Good point, but correct and incorrect.
Google Play Store 4.4.22 still working fine. It shouldn't be a problem to replace or manually update it to the latest
4.5.10.
Just download
com.android.vending-4.5.10.apk and install the update.
Bottom line is, the playstore has a convenient auto update feature which Slimroms disables. His brother would have to go to slimroms.net to download further playstore updates. Besides, Slimroms.net could stop supporting the ROM at any time and he would be "stuck" with an outdated playstore. It would work but he would have to pass on any new features or fixes. That's something to consider.
Second thing to consider, every ROM is based on Quarx ROMs. This means that every custom ROM is likely to have Quarx's bugs AND the bugs of the cutomized ROM.
4.3 performs well but it is not 100% functional. I've read they had troubles with bluetooth and I found that some options don't work (adding APN for instance).
4.1.2 is mostly functional but it's not 100% stable. I had several reboots on a fresh install.
Epsylon's latest 4.0.4 is still a good pick. I just installed it and it performs better than JB on Kingdom Rush (it stuttered, crashed and reboot on JB and worked flawlessly on ICS!)
CM7.2 is awesome in every aspects BUT it doesn't have HWA, which means he won't be able to use some of the latest app. Of course, it doesn't mean much since the Defy is not very powerful. But there are some good HWA only apps that still work on the Defy.