Consequences of Collagen Injections to Buttocks?

your friend can't be certain of the actual filler that was used. Whether collagen, or any other filler, substantial volume would be needed to create a visible improvement in an area that large. Once such sufficient volume of filler is inserted, there is a concern of migration, nodule formation, scar formation, necrosis (death) of the skin and ulceration (open wounds). Infection has occurred from improper technique and the use of non-sterile substances. Immune reactions have occurred from the use of products having adulterants (additives) or the product itself (e.g. paraffin waxes). Silicone reactions have been noted as much as 25 years after a procedure was done, so although your friend is OK now, she might not be "out of the woods" for many years depending on what substance was injected into her.
Remember, you are doing a cosmetic procedure to enhance the aesthetic nature of your physique and if a complication arises, even if it is of low probability, the aesthetic outcome could be disfiguring and permanent. If a complication occurs when you are treated out of the U.S., it is possible that your medical insurance would not cover the cost of the treatment of complications whether overseas or here in the U. S.

It is likely your friend had silicon injected into her buttocks.  This is permanent and I have seen many Latin patients that have had silicone injected into their buttocks.  For most of these patients, they do well initially, however with time the silicone becomes very hard and the the body rejects the silicone.  This can result in disfigurement from scarring.  I would advice you not to do this treatment. In the United States, there is no FDA filler approved for augmentation of the buttocks.  The most common way people have gluteal augmentation is from fat transfer to perform a Brazilian Butt Lift or buttock implants.  Although not FDA approved for gluteal augmentation, some U.S doctors recently started using a FDA approved filler called Sculptra for gluteal augmentation.  I have had fantastic results from Scuptra! Hope this helps!Dr. Sajan

Whatever your friend had injected into her buttocks in Mexico, I can assure you it was NOT Collagen - the dethroned wrinkle filler.
Collagen (now owned by the Allergan corporation - makers of Latisse and Botox) costs several hundreds of dollars PER 1ml (cc) syringe. Since it would take between 500 to over 1,000 cc of any filler to fill the buttocks (in the case of a Brazilian Butt Lift - FAT), the cost of the filler alone would be well over 100,000 dollars.
Whatever she had injected into her buttocks had not yet reared its ugly head. The worst may still be ahead.
Peter A Aldea, MD

As Dr. Aldea just said, there is no way your friend had collagen as this would have dissolved totally within a few short months. Some unknown and potentially dangerous permanent filler was used on her and someday may cause untreatable problems. She might get lucky but don't make the same mistake please.