use a zone defense when he has the ball in goal
put your two bar 21* man with the inside of the man on the dead man line (push side - close to you leaving deadman 1 hole if you will**) and put the goalie man in the same deadman spot on the long pull side leaving deadman 5 hole
leave your 5 rod basically on the middle dot and don't move it...use your 3 rod to stay about a ball or ball and a half in front of whereever you mate has the ball on the push or pull side
this is very easy to show but hard to explain...if you can find some video on youtube of Terry Moore playing singles against Johnny Horton you will see what I am talking about...there are ways to refine this but this is the basics
good article here with pics that will better explain it and give you some of the variations...you want to work with this on your own placing the men in different positions and then walk around on both sides of the table so you can see the holes that are open and how to cover them...if your mate is a beginner, he will find this very very difficult to score on outside of slop...I mean, Terry Moore (and others) use it at the highest level...it is VERY effective
http://www.foosballboard.com/fqzone.pdf* Number men on the rods starting with the man closest to you, it makes it easier to have these conversations...so on the 5 rod, the closest man to you is 51, second man is 52, third man is 53, etc...3 rod is 31, 32, 33...two 21, 22
** Break the goal down into 5 holes...the 1 hole starts closest to you, then 2, 3, 4, and 5 is the long pull side hole