Let's talk about one situation you surely have been through.
So, why is your client crying? During the procedure, not after one.

It often happens that your client's eyes start watering. Why is this happening?
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1. Sensitive eyes. Light sensitivity may be the reason. Try to reduce the brightness of your lamp (you do use a lamp with adjustable light intensity, right?) or stick a large patch on the eyelid, creating an additional barrier against the light.
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2. Reaction to the glue. Sometimes it is accompanied by tingling, and sometimes it's not! Check if the eyes are closed tightly. Even the tiniest slit between the eyelids allows caustic glue vapor to irritate the eyeball, causing discomfort and often redness.
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3. Eye fatigue. After long lying or because of the strong eyes tension (when your client is trying to control the eyelids to be tightly closed or tensely thinking about something), eyes can start watering. We need to help our client to sleep or take a short break for 5-10 minutes. Gently dry the place where the tears were, and prepare the lashes for extension in this area AGAIN! Yes, we degrease and apply primer.
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4. Dry eye syndrome. Eyes can begin to tear even before the extension, and nothing helps to stop the tears. In this case, it is better to advise your client seeing a doctor. It will be rather difficult to work with such a client, even if you place a paper tape on the corners. The tape will absorb the tears, but bottom lashes extension in the inner corners will not be possible, as the glue will instantly polymerize upon it touches a wet surface.
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In most cases, eyes don't water immediately, but sometime after the procedure. Therefore, I recommend first to work on the inner and outer corners, and then calmly proceed to the main part of the eye. Then tears will not impede your work.