I don't know about dying from loneliness. The above post is right, if he dies, it is because he is diseased, or heavily infested with parasites.
However, a big consideration is your own set up. Most backyards cannot really do an effective quarantine. It requires a great deal of space between the birds. One needs to change their shoes and clothing between caring for one set of birds and the other. Most hobby people are not set up for this. If you are just separating the bird with wire, that is not quarantine.
If you are getting a bird from another person, who does not go to swaps and auctions. Whose birds are well cared for, and the rooster is healthy looking and free from parasites, and you don't have a huge expensive flock, I would just add him to the girls.
Yes you are risking possible contamination, but it is drastically reduced in these circumstances. Personally I do not think it is good for birds to be separated, I just added mine a couple of weeks ago.
However, you can only do, what you are comfortable with.
Mrs K