welcome to hcm.
laxative may be of different types.
its not clear what types you are using.
no problem i will give you a idea.
bulk forming are safer.
irritants like castor oil may cause abortion
they increses uterine contractions.
so it should be avoided.
some others like bisacodyl
also should not be used.
but as there is repeated negative preg test after being positive she should stop abusing laxative.
isphagula husk is can be used safely.
so if she is using them she can shieft to 2-3 tea spoon full of isphagula husk at night.
consult a gynaecologist who is practicing infertility
no need to worry.
wish her a safe pregnancy