Consider the art they already own, is it all minimalist abstracts, or is it fun funky vintage posters? When you visit them next, take a quick peak at what they already own and try to buy artwork in a similar art style.
If their home is a modern or minimalist style, use a simple thin black frame, but if their space is more boho or neutral, go for a grey or maple frame.
Matching artwork to specific colours in their space is a great way to make sure it looks amazing in their home. Try isolating either colours of their furniture, rug or small decor elements and buying art in that colour palette.
If you notice they already own a lot of photography or line art, why not add to their collection? Looking at the art they already have hanging is the easiest way to make sure the art print you’re gifting them will match.