An contract can be made orally and there is no need for any agreement to be in writing. Indeed apart from contracts for the sale of land and certain consumer credit agreements which need to be in writing we make oral (and enforceable) agreements every day.
This is not a contract per se, but a gift. BF has already GIVEN GF a laptop. The text message telling her to keep it after the split is merely evidence which would support the fact that the laptop is a gift.
BF would be very stupid to report laptop stolen having sent that text message. At best GF will be able to defend it on the basis that she was of the honest belief it was hers by way of gift, at worst, as Sara says, he is committing an offence for which he could (albeit unlikely) be charged.
BF could try suing, but if GF has any sense she will plead that it was a gift and in evidence introduce the text message (as long as its provenance can be proved) .