The guy is a little weird but was always friendly to everyone. Should his colleagues collect something for him? Maybe a card with names on it flowers or something else? I'm not in his group and I haven't interacted with him directly much. Except for some smalltalk. It actually bugs me a little that the colleagues that worked more closely with him have not taken the initiative themselves. I work in northern Europe and the culture here is sometimes cold, but this seem odd to me.
My recommendation for an inscription for the book or card, if that's all you get, is something along the lines of:
When people only want to talk about your situation, you're allowed to say you're aware of your situation, know what needs to done moving forward, you're doing it, and that you would rather talk about their day or something else.
This unsolicited advice has been well received over the years.
If I were in that situation I'd send a get well soon card and flowers. I wouldn't call or visit. Usually the only thing you have in common is work, so you're left to speak about either work or their heart attack. The latter is not really your business and the former is something they shouldn't be worrying about right now, so keeping these gestures one-way is a nice way to say "we're thinking of you and hope you get well soon" without the risk of putting them in an uncomfortable situation.
Good luck, and I hope they make a full recovery.
Unless your coworkers are just heartless, the most likely thing going on here is a concern about not wanting to make it appear a big deal. Perhaps the person who had the heart attack, does not want to have everyone know about it. Some people wish to keep it quiet; they feel embarrassed. (My mother tried to dissuade me from informing even her closest friends!) And so dramatically highlighting their recent heart attack could be terribly distressing and awkward.
But don't get carried away. This is more of a thing for in conversation. Should they recover and return to the office, I would not bring it up beyond a simple "so happy to see you here and looking well". I wouldn't try to discuss it with them unless they bring it up first.
If the simple fact of the heart attack is in the open, then it is hard to imagine how someone could be insulted or distressed by a card, or flowers or similar note of well-wishes.