How do I accept untrusted Omemo Keys?

[JokerGermany]

Friend:
Ipad Mini with Zom

Me:
2Conversations
1 Gajim

Everytime when he sends a message, one of the conversations devices get the following message:
"The message was not encrypted for this device"
The other conversations device get the message.

The gajim device get the following error:
"[email protected] has requested an Off-the-Record private conversation. However, you do not have a plugin to support that.
See https://otr.cypherpunks.ca/ for more information."

This are the contact infos in his device:

[JokerGermany]

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