What kind of music do they mainly like to sing? It would help in figuring out what their needs are, some pedals have more special effects which would be good if your friend was into more experimental music, or have different delay chips used for the loops which can be more hi-fi or lo-fi depending on the chip and the clock rate they use and all that, some let you adjust those parameters. For general use I think it’s really hard to beat the Boss RC-505 or RC-202, they are well tested and have a good community around them as well.

The TC Electronic Ditto X4 is a lot smaller, it’s not quite as flexible as those but it’s cheaper and easier to use I think. If your friend is a little more experimental you can check out the Alter Audio Time Tosser which is dsp based and allows you to mangle your loop/samples a bit more. The Chase Bliss Mood is also worth a look if they are into lofi/warbly kinda stuff. You could also look into something like a TC Helicon VoiceLive Play which is a pedal effect specifically for vocalists and includes a looper, I have no experience with this one tho

If you’re not familiar you can go to reverb.com which is a marketplace for musical gear especially, but a lot of vendors sell stuff there new as well, you can find deals on b-stock and some good sales sometimes. If you want good support I would buy at sweetwater probably