One of our clients' membership account isn't working.  So eMember (our membership plugin) has suggested we delete that account and recreate it.  I'm wondering what affect that would have on their SP forum account?  I don't want to loose any of the information/posts they have made. 

depends on how that membership plugin works... 

if it fires the appropriate hooks, it will delete the simple press user info as well...  you should be asked what you want to do with the simple press posts they made...  options are delete, leave from guest, or assign to someone else...

if it doesnt fire the appropriate hooks, the wp user will simply be deleted and the simple press info will be orphaned with no user to attribute the posts too...

bottom line, there are only wp users... if you delete a wp user, its references into the forum info are lost...