Is It True That Several Years After Brazilian Butt Lift Your Butt Starts Deforming?

About 60% of fat from the transfer is retained and the results are permanent. The size will change with your weight just as the rest of your body does. In other words, the fat acts like it was naturally there. You can get a good idea of the final result after 6 months.

Weight loss and weight gain effects the results of Brazililan Butt Lift. 

I have not heard of gluteal CA after a transfer. Perhaps your friend can verify the source. Seek a plastic surgeon who performs hundreds of these procedures, has great reviews and photos, and is an expert in this field.

I have to agree with Dr Aldea
There has never been any reported cases of patients developing any sort of cancer due to the Brazilian Butt Lift/ fat grafting to the buttocks.
Any fat cell re absorption that will occur will happen during the first few weeks. If they are alive several weeks after the procedure, they will remain alive.
I have performed this procedure for over 12 years and have never seen any long term complications!

As we age - we will sag. Take the most gorgeous women of the 1970's and 1980's and look at them now. Aging is a part of life and the only thing that effectively stops sagging and aging is death.

The buttock is no exception. The tight round buttocks of an attractive woman in her 20's will not persist into the 60's.

As regards:
"...several years of the procedure the fat cells start to reabsorb making the butt deform and shapeless" - FALSE We all lose fat cells with age. The fat cells that survive grafting are nor different than the fat cells there now which WILL change with aging.
" it can cause cancer because alot of cells get damaged during this process.. " - FALSE I have yet to see buttock fat cancer. Liposarcoma is an extremely rare disease. You would think that with the number of Brazilian Butt Lifts done in the US and the rest of the world we would have seen a spike in such cases IF this was true.  Absolute nonsense.

I would rather shape and transform a woman's lower back with her own fat any day than by the injection of Silicone or Hydro gel into the buttocks.