The betta is a:
i) slow-moving fish
ii) with long flowing fins
iii) whom your friend keeps in a smallish tank (probably about 10-15 gallons?)
It therefore needs to share (if sharing at all) with fish that:
i) are neither aggressive or extremely fast swimmers (the latter would stress him out)
ii) are not fin-nippers or have long flowing fins themselves
iii) do not grow large and are not heavy waste producers
Of these:
i) rules out e.g. Chinese algae eaters and also fast swimmers such as danios
ii) rules out most tetras, probably danios
iii) rules out plecos
Corys are ideal in a situation like this: they are peaceful (i), do not nip fins (ii) and stay smallish (usually 2-3 inches). But your friend needs to check he has the room for a school ( 3 is a minimum, more is better).