Several things could be going on.
Does she have asthma? The change in breathing paterns of laughing can cause asthma attacks in some people. She could try pretreating with her rescue inhaler if she knows she will be some where where she will be laughing a lot.
Second, vocal cord dysfunction is a possibility. This is a spasm of the vocal cords that can be triggered by a number of things including laughing. You can do a search to find some basic treatments for this condition. They are mostly breathing techniques. The only treatment for this condition is speach therapy where the therapist teaches breathing techniques and a few other helpful things. Experimenting with this cannot hurt.
If niether of these ideas seem to fit, she may need to talk to her doctor about it at her next apointment. There could be some other type of muscle spasm that may be treatable.
Thanks for the Input.
I don't think she has asthma, as it only happens if she laughs too much. (I.e nothing else to indicate she may have asthma.)
An the second thing you mention could fit, but a breathing exercise wouldn't help if she started laughing too much because of something is really funny or constantly funny. Have you even tried not to laugh? It really hard.
I think probably asking the doctor the next time she goes is probably a good plan, but we are both forgetful, we get there and somethings we want to ask slip our mind.
An Update: Now I think about it, depending on how hard she laughs changes the amount of time before she starts coughing. Really hard laughing as a shorter time frame until coughing starts. But with lighter laughing she can laugh longer before the coughing starts.
Normally with lighter laughing it's not a problem as she has to laugh longer than normal for the coughing to start (I.e tickling her for too long)
I have the same problem as her, but mine was developed two years ago, after I was cure of pneumonia. Ever since then, when I laugh really hard, I start coughing and wheezing really bad.