My co-worker stinks up our washroom every day. Is there anything I can do to stop this?

My co-worker and I share a washroom in our office. I am her supervisor. We are fortunate enough to have a washroom to ourselves, but she goes in there every day at least once and has a very smelly bowel movement. She doesn't even try to hide the fact that she's going in there to crap. She'll make a comment like "Oh I have a runny tummy today" and then she goes in there for like 15 minutes. God help me if I have to pee anytime after that because I won't go near that bathroom. What really bothers me is that there are 2 more bathrooms down the hall that are more private. This is an office where there's a fair bit of traffic by our employees and sometimes, she'll come out and leave the bathroom door open and I get this whiff of crap. I don't want others to smell it because it's a disgusting way to welcome your staff. I don't crap at work and even if I absolutely had to, I would have the common sense to go and try to find a more private bathroom so as not to announce to the world that I am taking a dump. I don't go and use the other bathrooms because I am here all day, she is here 3/4 of a day, and I feel that I should be able to use a bathroom that's close by, especially because I don't make a day of going in there. She is a fair bit older than me (68) and will retire next December, but I guess it just bugs me that she has never adjusted her habits to take care of this at home or go and do it where it's less noticeable. I know I have to be careful with what I say, but I was wondering if there's a way to "drop a hint" about this. I find it disgusting.