I have a slipper sock pattern by Kim Guzman from a kit containing ready-made soles, size L womens. I want to make it for my BF and I was able to purchase the correct, size 13" soles on ebay – but how to expand that pattern??
Help! 
only 20 days til Christmas!
The first thing I’d try is going up in yarn size on the same pattern. Do a gauge swatch in a bigger yarn, and then check it against your women’s pattern. If it works great!
Just my opinion, and good luck!
While your friend’s foot is definitely longer, it may not be that much wider (unless she has really big/wide feet). You’ll likely have to adjust for length more than for width.
Is this the pattern or a variation on it?
http://www.knitandcrochetnow.com/wp-content/uploads/2010/09/3-112-Knit-Classic-Slipper-Socks-Designed-by-Kim-Guzman.pdf
It looks like you should be able to increase the width by adding maybe a stitch or two to the cast on and proportionately increasing the heel (as RK noted. the difference between M and L is in the length, not the cast on or heel sts). Then knit the foot for the required length.
Cute slippers and a great Christmas present!
Thanks Rochester and Salmonmac, my BF is my big boyfriend who is 6’4" and has humongous feet. They are sort of narrow for a guy but really high in the arch. I want him to be cozy in them not feel constricted… yes it is the same pattern 
I know how to work toe-up and fashion a faux heel flap and gusset for a high instep. Usually the stitch count for sock top and foot are the same so half the stitches you start with should be just fine for the instep. Based on that I’m surmising that you could knit the heel flap longer, pick up more stitches for the gusset, and just take more rows to decrease to the number of stitches needed for the instep. I think that just adding rows for length should take care of the length problem and the width shouldn’t really be an issue but if you’re concerned just go ahead and leave it wider when you decrease from the gusset. I’m sort of thinking out loud here so if this isn’t helpful, please say so. I know how I’d do it toe-up so I’m trying to convert it to top-down. I think I might just try these. Thanks for making me aware of the pattern!