Please beg her not to do it.
Not rehome if she must, I know that not all owners are feckless dumpers of pets, but rehome privately I mean.
Please perhaps print this out/show it to her. A decent rescue will do many, many things the owner can't for whatever reason. They will:
Assess the dog thoroughly and train/consult behaviourists as appropriate.
Neuter/spay and vaccinate, vet check and worm.
Assess potential owners in their own home, meet all the family and check all adults are on board with owning a dog, do follow ups and meet all existing pets so ensuring that the dog is less likely to bounce back to them and that owner and dog are suited.
Ensure, through written proof, that the new owner is allowed a dog - ie that they own their home or have landlords permission to own a dog on the property.
And, very importantly, insist by contract that they must take the dog back AT ANY TIME IN HIS LIFE if it doesn't work out or if they cannot keep him and have a no-kill policy to support this.
There is good reason for this. Damned if I can find it now but online somewhere is the story of a UK woman who rehomed her dog to what she was made to believe was a nice, responsible family.
Not long after the dog was picked up as a stray, IIRC, in a terrible, terrible state, beaten and abused. The owner had been duped by the "loving family home". This is far from an isolated case and all too often owners are similarly duped and their dogs used as breeding stock in puppy farms, bait for fighting dogs, fighting itself or just kept in dreadful conditions and abused because once he has left the owner it is out of their control and knowledge.
Equally I have often been tasked with finding rescue spaces via vets who have had the unhappy experience of a visit from an owner who cannot/will not keep their dog and so takes him to be "put to sleep"... and of course not all vets are as scrupulous and moral, many WILL take that action without reaching out to rescue. This is happening all the more since the RSPCA no longer accepts dogs from the public... often people have, or think they have nowhere to go. It's hard enough for me, with contacts across the UK and beyond, to secure rescue places, especially for older, bigger or ill breeds and crossbreeds or for the ones with "reputations" such as Bull Breeds and Greyhounds.
If your friend rehomes privately and in 6 days, 6 weeks, 6 months or 6 years time the new owner cannot keep pooch he could end up on the vet's table whereas if he went into reputable rescue they would always take him back, no matter what or when.
Please, please believe me, I see and deal with this sort of awful stuff far too regularly and have done so for years. Please ask your friend to seek a rescue with a genuine no kill policy which insists upon all the checks and assessments I've mentioned above. Unless the dog has issues with aggressive behaviour I would say that the Dogs Trust is a good start. Another is Farplace in Stanhope, Co Durham, (a few miles west of Bishop Auckland), a tiny independent animal rescue who I know personally. They don't take in many dogs but may well know of someone reputable to refer your pal to if they can't help themselves.
Please ask her too to start her search now and not to put it off if she's sure she can't keep pooch as, as I said, places are hard to find.
I said I wouldn't take on any more cases like this, being so heart-sore about them, but sod it, if I can help secure a rescue place please say. I have totally no-kill policies as an AR supporter and I'm completely independent in what I do. I work for the dog, not for any particular rescue, not for the owner or prospective owner and not for myself and, of course, the decisions are all those of the owner. I'm only the middle-person, putting a suitable rescue with a space in touch with the owner, with the dog's needs, interests, welfare and safety paramount in all ways.