I've been keeping fish for a fair few years a long while ago. I've only just recently set up my 60L tank and I'm currently going through the fishless cycle. (About a month to go).
I've inspired my mother though, she's got a 30L half hexagonal from Pets At Home and thankfully she didn't buy fish, since she wanted me to set it up! Again, in about a month or so she'll be wanting some fish. I'm thinking a single Betta so as not to over stock, keep the maintenance fairly low and it's something pretty for her to watch. However I fear that she'll want more than one fish...
I know well enough that 1 Male Betta should be kept alone. I can't think of many options for her tank other than the diddy fish. (And nobody likes telling their mother they're wrong). Is there 2 or 3 fish that could comfortably live in a tank of this size together?
Bear in mind, it's a half hex tank. It's as tall as it is wide so I know the options are slim to none so I thought I'd give a shout in here.
...and, if she insists she wants "more than one", you could offer the option of shrimps and snails *instead of* the one male betta fish. Maybe a planted tank with some nerite snails and some shrimp might be an attractive alternative to the betta fish.