I used to have a car with najis leather, and sometimes my friend would ride the car with me when it was raining so their jackets would touch the leather and become najis.

Now I have a new car that is not najis, if my friends come and sit in my new car with their same jackets, do I have to consider the seats as najis (since the jackets are mutanajjis)? Or can I consider their jackets tahir as there might be a small possibility that they have washed them? Or is it obligatory upon me to ask them and make sure they wash their jackets?

First of all leather in cars may not be najis. You have to know for sure that it is from a najis or animal that is slaughter in a non halal way to consider it najis. If you do not know that you consider it pure.

If you know for sure that it is najis, then their jacket has also become najis.

If it is najis then it will make a second item it touches najis if it is moist. If there is reason to believe that the jacket has been washed… which is most likely the case, or it got rained on and purified; you can consider it pure.