[susan53]

There are two things that need to be understood here:
a) the meaning of "some"
b) the meaning of the word order

"Some" here is understood as meaning "excellent/incredible", or in some way "extreme". With ordinary word order the meaning is literal. So after eg a wedding where the food was really good, a guest might say :That was some meal! (subject -verb -complement) with the meaning That was an excellent/incredible meal! or after an exam a student might say That was some exam! meaning That was a very hard exam.

If you change the word order, however, with the complement first, then it becomes ironic and therefore critical, suggesting you are disappointed, annoyed or just surprised that it wasn't as you expected. So if at the wedding the guests had only been given a small sandwich, they might say :
Some meal that was! meaning That was a terrible meal!. And if the student had studied for weeks expecting the exam to be really difficult, and then was only asked three really easy questions, they might say Some exam that was! meaning It was a bad, inadequate exam.

Like all ironic statements, they can used humorously though, as possibly in your example Some pal you are! meaning You're not a good friend. The speaker could be joking or serious - only their intonation and body language would tell you.

The use of "some" isn't essential. The same thing happens with expressions like :
A great help you are! (eg if someone has offered an obvious solution to a problem)
A wonderful summer we're having! (eg if it has rained solidly for weeks)
The word order (C-S-V) in both these sentences signals that the apparently positive evaluation indicated by the adjective should be understood ironically, so that the meaning is actually the opposite: You're no help at all /We're having a terrible summer.