As always, you never know until you get them together, and it greatly depends on the betta's personality.
Neon tetras are bright little fish, and there is the remote possibility they could provoke an attack from a betta. However, neons are also quick, schooling fish, so it’s hard to imagine he’d land any punches.
Personally, I have kept a betta in a large tank with a school of about 15 neons with no issue. That doesn’t mean there couldn’t be a problem, though, and if you try it be sure to have a backup plan. As it was then, my main concern would be the betta frantically and constantly trying to chase down the neons, and stressing both the neons and himself.
A secondary concern is the neons nipping at the bettas fins which, while unlikely, they may try. You can see the danger there is to the betta, not the pint-sized neons. This is often the case when you try to keep a betta in a community tank – the victim is often the betta.
If you try this, I advise at least a ten-gallon tank, but bigger is better. Neons should be kept in schools of half a dozen or more, so you need a tank at least this size. Most importantly, have a backup plan. You need another tank or a bowl so you can remove the betta if things go wrong.
Here is an article you may find helpful for more advice on how to keep a betta fish in a community tank: