Hi jj, I had a colostomy bag (an ileostomy rather than colostomy) for five months, many years ago. There is so much support available online, including Facebook groups and other websites. Colostomy UK has already been mentioned but I'd also suggest Ileostomy & Internal Pouch Association:
https://iasupport.org/ They have a forum for asking questions as well. People will have different experiences with a colostomy bag but I'd recommend her talking to someone who has been through it, either online or in person, to get some reassurance. Also, the hospital will have stoma nurses specially trained to help through the whole process and beyond - and in my experiences, these people are a godsend.
If you or your friend has any specific questions or worries I'll be happy to try to answer based on my experience. Sending best wishes to her.