Should I tell a colleague that he's serving as an editor for a predatory journal? A colleague of mine, who I regard scientifically, just invited me to submit an article to a special issue of a journal which I consider predatory.
I consider this journal predatory because its publisher released several journals that are considered predatory in my scientific community (bioinformatics), and also for some details like the super-fast review and acceptance process.
The last article this journal published, for example, has the following review and acceptance times listed:

Submission: 12th September 2020
Revision: 23th October 2020
Acceptance: 28th October 2020
Publication online: 29th October 2020

This aspect is very suspicious to me.
What should I do now?
Should I tell him that I think the journal is predatory?
In this case, he might get offended, and this situation might compromise our future relationship. Like, he might not want to collaborate with me anymore in the future.
But if I say nothing, the scam could go on...
First, you need to have very solid reasons for considering the journal to be &quot;predatory&quot;. I bring this up because the only concrete reason you gave in your question is a rather poor one: rapid peer review cycle is not necessarily evidence of being a predatory journal, even if it might be unusual in your field (as it is in mine). My point is not to dispute your assessment (I have no idea of what journal you are even talking about); my point is that if you would consider discussing it with your colleague, then you should certainly focus on explicitly disreputable behaviour. In particular, the publisher's journals should clearly be listed on at least one website that tracks disreputable journals. That way, it is not just your assessment; you would have others who would support your concerns.
That said, a publisher with some disreputable journals might have some very genuine journals in its portfolio. This is a very important point: you cannot automatically denigrate one journal just because of other journals from the same publisher. And I repeat, a rapid review cycle is rather weak evidence. Stronger evidence would be actual access to review reports that you can examine and judge to be of unusually poor quality.
So, this brings me to your direct question of whether you should share your concern with your colleague. If you are afraid that she might get offended, then it seems that your relationship with her is not sufficiently close that she would trust that your concerns are personal concern for her. In that case, it would be especially important that you have strong evidence of disreputable behaviour before accusing the journal she is involved with of being predatory. If you do not have such strong evidence, then it is best not to mention anything but to politely decline the invitation with a truthful but evasive answer (e.g. &quot;I am unable to commit the time to such a submission&quot;--this would be truthful since you are unable to waste your effort and time on predatory journals). But then, even if you do have sufficiently strong evidence to accuse the journal of being predatory, you would need to determine how comfortable you are with this colleague. Only you know that, so ultimately it would be up to you to decide whether she might be offended by your presentation of evidence.