I am assuming that you are familiar with LDAP queries and so I will skip all that (except, of course, to remind you to wrap your Searcher, etc in using statements for proper disposal <g>)
Basically what you are going to have to do, I am afraid, is retrieve that person from AD. The nice thing is that you are given their CN, so it is a quick retrieval. Just have the PropertiesToLoad for that Searcher be their userPrincipalName and VOILA!!!
;)
give me suggestions for what to blog...
http://www.myfriedmind.com/techblog -> thx
Mark as "Answered" if this solves that wee old problem...