Send via SMTP instead of using the client? You could store SMTP settings for each user of the database in a settings table, and grab the settings to make them global each time a user logs in.
Use an email plugin instead? From my experience, the plugins out there are just a lot more reliable.
This plugin is specifically for sending outlook mail:
Unfortunately, I've also run into this, I do not think Filemaker is to blame, Outlook is highly tempermental and we almost never use send mail via system client as a result. You might want to see if her outlook is locking up for another reason (IE she has a huge mail file, or has a system that's on it's last legs, outlook is a resource HOG)
A client pointed out a very similar issue on 31 May 2013. Using FMP12 v3 and Outlook 2003, the email message is sent from FMP to Outlook. If the client hits "Send" in Outlook all is well (no issues). If the client tries to edit the email message (add/remove one character), the Outlook application exits, except for the email message window. I would have thought the rogue email window NOT possible until I saw it myself.
Quick workaround is a layout with some global fields allowing the client to edit the email message before the send to Outlook. No editing in Outlook allowed, only click the Send button.
Longer term solution will be to build a Correspondence table in the database and use the Outlook Manipulator plug-in (Productive Computing) to push/pull correspondence to/from Outlook as required. The only hurdle I have encountered with the Outlook Manipulator plug-in (Productive Computing) is it does NOT work with x64 bit Outlook installations. This client is OK but for another client this has been a showstopper.