It depends on how long you want it to be. If you can split your yardage in half then you just need a few inches more than halfway around your crib (for seams/hems). If you want it to be longer, then you'll need enough to go all the way around.
Most fabric is 45" wide. Upholstery fabric is often 54" or 60" wide. So, it depends on your fabric width. It also depends on the drop (how far it hangs down) of the skirt and whether you want it gathered or not. You also have to add seam allowances and fabric for the deck of the crib (where the mattress goes).
I am assuming your friend isn't working from a pattern?? My recommendation would be to work out with her how she is going to make the skirt (design), how much of a drop you want, do you want it gathered, and what she normally uses for seam allowances and let her tell you how many yards of fabric. If she sews, she should know how to figure it out.
Here is a really basic pattern you can get an idea from but if you want a different drop, multiple fabrics, box pleats, borders, etc.... you would need to modify it:
I believe that the pattern I used called for 3 yards of fabric for the skirt and 2 of muslin for the deck (the part between the mattress and crib frame). I modified the pattern to only have three sides since the crib will be against the wall and now have about a yard left over which I plan on using for a quilt. That said, my skirt has flat "leaves" rather than a ruffle which is shown in the link above, so you'll def. need much more fabric if you're making that style - When sewing, I typically buy what the pattern calls for and a little extra just in case.
Re: For those making their own bedding or know how
Most fabric is 45" wide. Upholstery fabric is often 54" or 60" wide. So, it depends on your fabric width. It also depends on the drop (how far it hangs down) of the skirt and whether you want it gathered or not. You also have to add seam allowances and fabric for the deck of the crib (where the mattress goes).
I am assuming your friend isn't working from a pattern?? My recommendation would be to work out with her how she is going to make the skirt (design), how much of a drop you want, do you want it gathered, and what she normally uses for seam allowances and let her tell you how many yards of fabric. If she sews, she should know how to figure it out.
Here is a really basic pattern you can get an idea from but if you want a different drop, multiple fabrics, box pleats, borders, etc.... you would need to modify it:
https://sewingneedlework.suite101.com/article.cfm/how_to_sew_a_crib_skirt
If this isn't helpful, send me a message and I can give you a really detailed instructions on how to figure out your yardage.
Heather
I'm planning to make my own crib bedding... I'll be using these instructions:
https://sew4home.com/projects/bed-linens/355-stylish-baby-nursery-crib-dust-ruffle-
They have a lot more instructions too for other things in the nursery too