If the boots you purchased had been lost or damaged and therefore weren't available for sale then a decent seller would have contacted you beforehand to explain the situation, apologising for the inconvenience caused and would have then refunded the money. In such cases the item would not have been relisted for sale again, so it's clear that this seller was obviously not happy with the final sellilng price and subsequently cancelled the sale. However, that's no excuse - once you'd paid for the boots you were the legal owner of the boots, and so the seller had no right to re-advertise them for sale again.
The best way to deal with sellers who take your money and then renege on the deal without a justifiable reason for doing so, and who then exacerbate matters by refusing to refund the money, is to open an Item Not Received case after eBay's latest estimated date of delivery has passed, making sure that you escalate the case to eBay if need be in order to secure a total refund. Once that's been dealt with leave the seller some negative feedback in relation to the item so that other potential buyers can see exactly how this seller treats his/her buyers.
If your friend hasn't already done so then advise her to also leave the seller some negative feedback given the fact that the seller has reneged on the deal with her as well. If the seller receives too many negative feedbacks in relation to refusing to proceed with the sale after buyer has paid for the item then sooner or later eBay are going to notice that something is amiss, in which case these cancelled transactions may well end up backfiring on the seller. He/she will come unstuck eventually if this type of behaviour continues, especially if he/she receives too many defects on his/her account as a result, not only from negative feedback but also from eBay having to force a refund if the seller refuses to refund the money voluntarily. If the seller is a repeat offender and racks up too many defects then eBay may well suspend the seller's account and prevent him/her from selling on eBay. The seller may well believe that he/she is untouchable, but sooner or later something will happen that will shatter that illusion and bring the seller to his/her senses.
