The problem with asking a doctor is they will err on the side of caution. So if they don't know, they'll say don't do it. Asking the doctor is probably still a good idea, but if the doc says no but doesn't seem to have a lot of knowledge about it, I wouldn't take that as a final answer.

From my totally non-expert understanding, birds and reptiles are both possible salmonella carriers. So if she's keeping a pet bird, a pet reptile probably does not pose a great increase in risk. If she brings raw chicken into the house and handles it while preparing it, that is a risk too.

If I were in that situation, I think what I would do is get the reptile I wanted, but take some extra precautions, many of which she is likely already doing just as part of living with a weakened immune system. As Becky mentioned, she could wear gloves when cleaning the cage or even when handling. Based on the pics I see on here, most people don't worry about letting their snake roam on their bed, couch, etc. But with a weakened immune system, I might choose to limit what the critter is allowed to come in contact with to surfaces that are easily cleaned and sterilized. I might go a step beyond washing my hands after everytime I touch the snake or the enclosure and also change my clothes if my critter touched them.

Of course a lot of this depends on just how weak your friend thinks her immune system is.