I failed to ask, what version, so it would be Lotus Notes (LN) version??
I did find these: Notes/Domino 6 and 7 Forum : RE: Excel attachments will not open in Notes client
Where you would have to check the Incoming mail preference settings in the user's Person Document in the Server Address Book.
And: IBM LO55799: VIEWING DOCX ATTACHMENT DOES NOT OPEN WITH CORRECT APPLICATION - United States
This sounds like you may have a problem with the way Chrome handles the attachments. IBM LO50307: CERTAIN ATTACHMENTS DO NOT OPEN IN DOMINO WEB ACCESS (DWA) - United States
This is to be fixed in the next update.
Which leads me to ask, If you, and your others version of LN is lower than 8.5.3 you may want to consider updating to it.
This could be an association problem.
Which ever version you use is it Lotus Basic or Lotus Regular?
In your LN go to Location Preferences, under Internet Browser, don't select whatever is listed, instead select others and then browse to the path of Google or Chrome, and select:
"C:\Owner\Local Settings\Application Data\Google\Chrome\Application\chrome.exe
" or if need be just chrome.exe
Then save it.
Now your links should always open in different Chrome window.