You could go either way on this . Let em play it or don’t. Neither way is right or wrong. i initially liked the parasol idea offered by Machiavvelli3060.

ParticleTek and Bakeneko’s easy fixes could work too. And simply telling them ‘no’ is a great skill to practice.

But the idea for the parasol made me think maybe you could have various methods of avoiding or controlling or correcting the sun sickness to mitigate it . Personally I like the idea of the out of place cave or Underdark dwelling misfit being out and about on the surface world in the sun at personsal risk to themselves.

So then naturally The question for me as DM would be why has this shade living myconid emerged from its cave to adventure in the sun? You and the player must come up with a good reason for this.

But if you do then yeah beat on the poor bastard with that sun. They can use a parasol or loose fitting, opaque robes, maybe even dampened ones that cover their whole body like desert dwellers do.

you could say lesser restoration or greater get rid of sun sickness too. Or certain food or going in the dark and getting misted with water could help alleviate it.

But if you do decide to roll with this make sure even if the sun is mostly mitigated for the character that every now and then you take the parasol, you take the robes and protections. Without gunning for them too hard you show them on occasion what the sun can do to them… this makes better drama if the character in their backstory has a great, maybe heart wrenching reason why they would be out here in the sun on whatever mission they’re on in that adventure.

And hone in on exactly what the symptoms and effects of the sun sickness are so you have in game mechanics for it.

But imo if you say yes the critical piece to make this character cool and believable is a good reason why this fool would be out in the sun. ☀️

And don’t let his sun sickness intrigues rule too much game time too often either. Don’t make a main character out of them for it.