I’m not sure what checking her phone would achieve,
you know she has mental health difficulties. There are accepted routes to treating those - so if she has anorexia the nhs will offer specialist eating disorders service which usually includes working with parents, if it is OCD then CBT therapy is generally recommended.
it can be very tricky getting access to these nhs services these days do more and more people are accessing them privately.
fir many MH conditions medication can be useful and again nhs services are creaking so I avcsssrd (as many others do) a private psychiatrist who prescribed for my DS.
most MH conditions benefit from general be healthy advice - get outside at least once a day, try to keep sleep to night time, eat healthy meals etc and so most parents will try to do that.
there is a fine line on many MH conditions between supporting the teen and pushing them just a little bit further on that day if you think they can take it - eg encouraging them to go outside for a walk, enccouraging them to eat a bit more, etc.
teens of any description react very badly to attempts to control them and teens with MH issues can have particularly strong reactions. It’s not a good idea to do anything that looks like attempts to
imposr control - eg checking phone - without at least discussing it first and agreeing it with them
there’s also the issue of what will you do if you find eg she has been writing about self harm or encouraging others to self harm or emailing the Samaritans or stuff like that?
she’s 19. It’s not ok to do this.