Cross contamination is a key issue for Coelics.
For Coeliacs, gluten avoidance is nonnegotiable, & often lactose intolerance is a sister act.
So, suggest you oh-so-politely enquire I’d Dad’s new LF is Coeliac or if her gluten & lactose avoidance is an expression of choice. Ask your dad, by this stage in their relationship he will prolly know. Although with the slightest doubt on his part, then ask LF directly.
A reasonable question for a host to ask. If you position the question as an enquiry to determine if cross- contamination is a priority, if she is Coeliac she will really appreciate this. She may offer to bring something to save you some bother (do accept) and whatever GF/LF you make for her (& everyone else), keep her portions & plates carefully aside & clearly labelled.
If her gluten & lactose avoidance is an expression of choice, then cater as GF/LF as best you can, but cross-contamination is not such an issue.
Nothing as annoying as a welcoming host (moi) to find non-Coeliac guests have scoffed GF guests carefully prepared food leaving them with next to no food that has not been served up with mixed plates and serving spoons. Grrrrr.