I don't think your friend should worry about looking like a "jerk" for giving a gift card in that amount. I can see why that might seem small if the couple and all of their friends and family are wealthy. Maybe that's the case here. But for me and most people I know well, that would be considered a nice housewarming gift.
Last Christmas, someone gave me a bottle of white wine. This person drinks wine with their dinner most nights, and raved about how it was one of their favorites. I hardly every drink alcohol, so it took about six months for me to use it when I decided to make a recipe that called for it. I also served it to drink with the meal and it was delicious, so I made a note of the name.
A couple of months later when I wanted to make that dish again, I went to buy the wine and found that it cost $6! I never would have guessed it was that inexpensive and it was a very nice gift indeed. So a $20 gift card is worth more than three bottles of that wine.
Of course, I also think it's fine for the friend to just not attend the housewarming at all.
I understand that it can be overwhelming to keep buying gifts for the same couple over a short span of time. I know a young woman, "Susan," who had a baby and broke up with the child's father shortly after baby was born, started dating someone else and was expecting her second baby to be due close to the first child's second birthday. The children are the same gender. Susan and her boyfriend were living with relatives but were able to move out into their own apartment shortly before the second baby was due. They also decided to get married at the courthouse with only their parents and grandparents present, but sent out an announcement with photos via email.
Within about a six or eight week span, they had the following events: Baby shower for the second baby ("Because it's my boyfriend's first baby!"), oldest child's second birthday party, housewarming party, and wedding. I don't remember the actual order of those events and I didn't attend any since they live a four-hour drive away. I sent a gift for the birthday party along with one gift to serve as a combo housewarming/wedding present, and sent the newborn a present either before or after the shower since I was going to send them one anyway.
This couple had their first anniversary a few months ago and sent out a group email asking that since their friends and family know they didn't have a "real wedding" or honeymoon, maybe we could chip in $50 each so they could have a honeymoon now?
I don't think that worked out well, because about a month after that, they sent out another email stating that since that seemed too much for most people to contribute, maybe we could each give $10 because they really need better pots and pans. LOL
I was already trying to figure out what amount in toy store gift cards I wanted to send for each child's birthday when I received that first email, and that's all I sent.