What can I cook for Indian guests without trying to mimic Indian cuisine?
If Belgian food is anything like the Dutch food my Oma made, your best bet is to limit it to the dessert course and strike out in a different direction.  Even there, throwing in an extra pinch of salt and a little fresh ginger or cardamom powder may help liven it up for your friends.

One area where you'll both be happy: Mediterranean/Middle-Eastern fare.  If you pick a recipe with plenty of aromatic herbs and spices, and maybe yogurt, it'll have the rich flavors your friends are used to. However, they won't be the same dishes they have at home, so they can't compare unfavorably to their mom or auntie's cooking.  Flatbreads, kebabs, braised lamb, goat, yogurt sauces... these will be familiar elements, but by using Mediterranean or Middle-Eastern versions, you'll give them a taste of the exotic too.  

Using South or Central American cuisine will have similar results; dishes rich in pungent chilis, cumin,  and coriander will combine elements of comfort food with something new.  Indian chapatis and naan (flatbreads) are very similar to homemade tortillas.  It goes without saying that beef is to be avoided.  Depending on what part of India your guests are from, coconut milk and tamarind paste may be other familiar elements to play with.  

Portugal also has a fine tradition of spicy foods, including the Vin d'alho which Indian adapted into the now-familiar vindaloo.

If you don't feel comfortable with these cuisines, you might do Indian, but with a fusion twist.  I found that Saag Paneer with blocks of feta instead of paneer is absolutely divine.  Halloumi cheese might work well too.  In fact, I'd say it's better than the original dish.

A word on dietary restrictions:
Presumably you know your friends' dietary needs, but if they're bringing new acquaintances it is wise to give some thought to dietary restrictions.  Many Indians are Hindus, who do not eat beef, and another major fraction are Muslim.  Muslims follow a dietary law called halal, which is very similar to kosher law, and likewise excludes pork and animal blood.  It also excludes alcohol... which means a spicy tamarind-port pork marinade isn't viable. 
A sizable part of the Indian population is also vegetarian, so any meal should include a vegetarian entree.