Perhaps Demoman could express his interest in Ilse when Heavy mentions that he met her earlier that day? (While Medic isn't there, of course.) That would easily motivate Heavy to persuade Demoman to give it a proper go with her but also cautioning him to be careful and not to be too pushy in case Ilse isn't interested in return. It would, naturally, be easier if Ilse already had an interest in Demo and also entirely possible, especially if Demoman had been flirting with her for awhile and she simply hadn't given in yet out of a misguided sense of loyalty (or simply for fear the rest of the team would find out and mock Medic or else figure out why Medic was ignoring her.)