I wouldn't do this if I were you. I'd be keeping Karlsson over Seguin (and over Backstrom too). You have Crosby, Ovechkin, Karlsson and Quick - all arguably the best players in their respective positions. Your keepers can't really get any better than that and I wouldn't be looking to make any moves. I don't think it makes sense in your league to keep two centers. Backstrom is much easier to be replaced through the draft than Karlsson is. It's not really even close. Karlsson should be worth much more.
That would certainly help your friend though. Karlsson and Seguin would be his two most valuable keepers and both are more or less equal to Backstrom (Karlsson more valuable, atleast to you since you already have Crosby as keeper C, and Sequin really close to being as good as Backstrom). They'd be much better keepers for him than any of Hossa, Datsyuk or Luongo are.
But like said, I wouldn't do this if I were you. I'd be happy keeping Crosby, Ovechkin, Karlsson and Quick.
UHL Pittsburgh Penguins
24 Team Fantrax Salary Cap H2H Dynasty League Daily
G, A, +/-, PIM, PPP, SHP, GWG, SOG, Hits, Blks, FOW
W, GAA, SV, SV%, SO
23 Man Roster - 3C, 3LW, 3RW, 3F, 6D, 1G, 4BN, 4IR
27 Man Minor Team
C- Tavares, Stepan, Hodgson, Shaw, Bjugstad
LW- Ryan, Perron, Downie, Killorn, Paajarvi
RW- Wheeler, Simmonds, Coyle, Frattin, Rinaldo
D- Schultz, JJ, Bogosian, EJ, Josi, Cowen, Muzzin
F: Granlund, Silfverberg, Jensen, Dalpe, Bowman, Knight, Fasth, JAM
D: Beaulieu, Koekkoek, Gelinas, Pysyk, McCabe, Irwin
G: Nilsson, Nilstorp, Ramo, Berra, Brossoit, Gibson