I think there are probably a lot of vitamin/mineral options that could affect mood and stress/anxiety, I think it's a big subject.
I was having poor quality sleep and used Root for a while. It has the calming effect of a mild benzodiazepine and is sometimes used to wean people off them and alcohol. It is available in health food shops here in the UK. The effects seemed to last longer than when I was in bed as well.
I was reading on here, this morning (GMT ), that GABA will help with anxiety (if that is what you mean by stress as 'stress' can be interpreted many ways). GABA is probably a bit overkill if she just wants to relax a bit maybe. Someone with better knowledge than me should probably comment on that one though.