My partner had with a worker, should i use a condom?

My regular partner is in an open relationship and has only slept with other people who have been tested. Normally we do not use condoms as I've had a vasectomy and we are fluid-bonded. This weekend, my partner visited someone they'd been developing a relationship with and had with him. This friend has also been actively working as a worker. They have been tested for other STI's but not for HIV. Now...my partner did use a condom while having with this person. My question, though, is - should i use condoms with this partner until they are out of the window period or is it okay to have sex without a condom because they used one with this other person?