It only has the second meaning - they can become friends. For the first meaning, you can use may, might or could. They may/might/could be friends. For might or could, it's not clear if it means they're friends now or they can become friends in the future. Can only tells me something is possible, so it has to be about the future. They can be friends. = They have permission or an opportunity to become friends. Could tells me it's a present or future probability. They could be friends. = Maybe they are friends or maybe they aren't, either now or in the future. I think can is the most difficult word in the English language. If you want to learn more, you can look up modal verbs and modals of probability and try to see the difference between permission, opportunity and (physical) ability.