In most cases, internet upload speed is less than download speed. So your position uploading to you friend might have been lagging alot when his position was downloading to your computer at a reasonable rate. What is your internet speed?
How did you know he couldn't see you. Were you using the LFS in game chat or something like MSN or Teamspeak. Disable all other internet programs when using LFS to decrease your latency which might solve your problem.
Perhapse your ISP was having a bad day and your upload bandwidth was slower than normal. Does this problem always occur for you? Perhapse your ISP uses packet shaping to curb Peer-Peer sharing like limewire/emule type programs. You might want to mention your ISP plan limits and the ISP you are with. Someone else here might know whats wrong straight away based on that info.