CandyCustard said: If its an allergy I'd play safe with soya custard and Hec/natrosol type gunge. Obviously no nutella either
Like Candy said don't chance it. I had an ex with Celiac (Coeliac). They were SO sensitive to gluten that even touching bread would cause them to break out horribly.
Maybe they have a tolerance level, but let them determine if they want to test it out or not.
Even if it turns out that they don't have a bad reaction to skin contact, there's always the chance of swallowing some.
I think the fear of it would spoil the fun - far better to follow the advice above and avoid anything that might trigger a reaction. That way you both get to lose yourself in the moment without worrying 'what if'.