Welcome to the forum
It might be worth mentioning your rough location (particularly US/UK) as this may affect the likely value. I see current stockists offering £25/bottle for a case of 12 at retail, so Alex's estimate that you could sell at £15/bottle for the case sounds reasonable; this could be varied up or down slightly depending on the bottle condition and how/where they have been stored. Also don't be too scared by the first line of Alex's comment:
AHB wrote:Malvedos 1990 is not a first league port and isn't what would normally be regarded as investment grade.
While perfectly true (since 1990 was not a 'declared' year), this is still good quality vintage port and should be very pleasant drinking. You could always open one and try it with your friend (if you do, make sure to filter/decant properly) and decide whether to keep some, all and decide on a fair price that way for any you decide to sell.