She got burned and it left scars; does she have to tell her fiancé?
If the scars from the burns are large and disfiguring in a way that could be off-putting to the husband, it is essential to tell him before doing the marriage contract, because withholding that information is regarded as deceit. The basic principle is that any defect that could be off-putting to the husband and prevent fulfilment of the aims of marriage, namely compassion and love, must be disclosed and it is not permissible to conceal it.
This has been discussed previously in the answer to question no. 103411
But if the scars are not very visible or they are unlikely to cause the husband to be put off, then you do not have to tell your fiancé about it, because that is not regarded as one of the defects that would affect the marriage.
In the event that it is obligatory to tell the husband about it, that could be done through your father or brother, so that you can avoid talking to your fiancé unnecessarily, because he is still a non-mahram to you like any other non-mahram.
We ask Allah to help and guide us and you.
And Allah knows best.