$4200.

.25 Gross Income Limitation for a Qualifying Relative. For taxable years beginning in 2019, the exemption amount referenced in § 152(d)(1)(B) is $4,200.

https://www.irs.gov/pub/irs-drop/rp-18-57.pdf#page=19

Does mom live with the taxpayer?  If so, I think it would qualify if the taxpayer paid for over 50% of support (but with $18,000 of income, that is questionable), even if her income is over $4200.

Publication 503:

A qualifying person is: ... A person who wasn't physically or mentally able to care for himself or herself, lived with you for more than half the year, and either:

1. Was your dependent, or
2. Would have been your dependent except that:
   1. He or she received gross income of $4,150 or more,

https://www.irs.gov/publications/p503#en_US_2018_publink1000203267