Hi @1Wanda1
Firstly I'm sorry your friend is having to undergo treatment, but it will mean a lot to them that you've sought to find this stuff out.
A bone marrow transplant or stem cell transplant (same thing) isn't an operation. The procedure itself is more like a blood transfusion.
What happens is that they give you some very strong chemo, usually over a course of a few days, which will kill off your own bone marrow (which is where blood cells including cancerous cells are made).
The chemo makes you almost completely neutropenic, which means you have almost no neutrophils in your blood to fight infection.
Without an infusion of new bone marrow cells, the chemo would leave you so weakened you would be at very high risk of death. But the infusion of stem cells (which are given like a bag of blood) is effectively a rescue or like a system reset. The donor cells travel in your blood stream to your bone marrow and begin generating new cells, effectively starting a new immune system from scratch.
After about 7-10 days blood tests reveal that neutrophil levels have started to rise meaning the donor cells have taken.
After the chemo/stem cell infusion, you are moved to an isolation room which is kept as sterile as possible. You are allowed very limited visits and everyone who enters has to wear PPE . You can be in this isolation room for a few weeks until your blood levels improve enough for it to be safe to go home.
Most people feel pretty wretched for the first week or two while the chemo effects play out - the side effects are unfortunately pretty brutal.
Recovery is slow and it takes several months to get back to feeling somewhat normal. It can affect your energy levels long term.
There's very good info about the whole procedure on the Lymphoma Action website.
I wish your friend an easy ride, good recovery and success with their treatment. It may be three months later that they get a PET scan to assess the success & whether or not they've achieved remission.