So, here's my perspective, as someone who started my journey into nonmonogamy through swinging. Your wife is honestly just asking for heartbreak if she pursues this couple. (Obviously, this is all my opinion.)
First-- they're comfortable asking her to date independently. Are they going to each be dating her solo, or are they expecting it to be a trio all the time? If the first, maybe it'll be OK. If it's the second, eww.
Second-- have any of you done any of the work to get ready for polyamory? Sex is one thing, love is another. Jealousy and envy are beasts that easily kill relationships all the time. What have ALL FOUR of you done to be ready, especially your wife, who is going to become a hinge partner between her spouse and not one, but two others? Is she prepared to balance needs? Communicate honestly and openly all the time with everyone, while also respecting people's privacy and trust?
Are there kids in the picture? If so, what will they be told? What about friends and family?
Third-- what do you mean by safe? Emotionally, who knows? Physically, as long as everyone involved gets tested and has a record of behaving decently, I don't see why it wouldn't be. This is another area where I ask if y'all have done the work, though. Do risk levels and risk tolerance line up? Have things like testing practices and schedules, pregnancy prevention, if it's a concern, safer sex practices, etc., been discussed? If pregnancy is possible, what is the plan in case of an accidental pregnancy?
Fourth-- if your wife does choose to pursue this couple, are you all comfortable with you also trying to meet someone(s) to date? If yes, see all the discussion points above. If no, that's a breeding ground for resentment. Honestly, even if yes, think about how you'll all feel if wife falls happily in love with two other people, and you never meet anyone. If you aren't prepared to deal with that, then don't open the door to it.
Fifth-- it's on the table. You're all thinking about it. Is there a going back to the original plan or is there only an "I'm not comfortable with THIS situation, but we can figure our shit out and move towards poly in a different situation," or...?
Hubby and I opened up from monogamy, to swinging as a couple, to completely sexually open/"playing separately," to polyamory. A swinging partner that I fell for was the reason we became aware of polyamory and pursued it. That relationship was NOT healthy. We had not done sufficient emotional labor and were not prepared for many of the issues I outlined above. It sucked. I don't recommend it.