My daughter has always found tampax to be best, she 19 and used them from about 13/14. They do a teenage selection, I went to boots and got her a starter kit which had a little carry case and some wipes and things in, don’t know if they still do it. Anyway, word of advice, start her small size, and wear a pad with to allow for leaking, then build up to the size that suits. My daughter always used pads only at night, because I did, don’t know why now !, start her when she is at home for a few days if pos, to get use to them, and remind her to relax, if she can’t do it then go back to pads for a few hours and try again . My daughter now uses a cup, never used one myself, and not keen on her using it at first, but she swears by it now, been using it for a couple of years.
Oh last thing, make sure she changes regularly, it’s easy to forget your wearing them !.
Hope that helps., good luck