It is possible that Eanfrith had been…

It is possible that Eanfrith had been initially cooperative with Cadwallon.

The historian D. P. Kirby, pointing to Eanfrith's evident ability to quickly exploit Edwin's death, has speculated that "a wide-ranging set of alliances" that included both Cadwallon and Eanfrith may have existed. (Kirby, The Earliest English Kings (1991, 2000), page 73.)

If there was a friendly relationship between Eanfrith and Cadwallon at first, however, it must not have lasted, since Bede reports that Eanfrith went to Cadwallon "with only twelve chosen soldiers" in an attempt to negotiate peace, but Cadwallon had him killed.