Advice please for lactose intolerant DD

[OhNoNotMyBaby]

DD has had severe cramping and diarrhoea intermittently over the years (she's 17 now), usually on Christmas morning after too much chocolate, sometimes after eating restaurant pizzas..

It seems to have got worse lately - which I blame on the stress of exams - not sure if stress makes it worse?

We are finally trying lactose-free foods, and she says she feels much better. (Except she lapsed with Galaxy chocolate yesterday, though she says it was the small dollop of cream in her scrambled egg...)

I'm asking for advice and other people's experiences. Does this intolerance come and go? Any recommendations for foods (Tesco seems to do a better range than Sainsbury's). TIA