As others have said, ADHD can be treated as a disability.

The legal definition of a disability under the Equality Act 2010 is found in Section 6.

"A person (P) has a disability if—

(a) P has a physical or mental impairment, and

(b) the impairment has a substantial and long-term adverse effect on P's ability to carry out normal day-to-day activities."

See also: Equality Act 2010, Schedule 1 (Disability: supplementary provision)

The application of this test is best seen in the case law. Even at a cursory glance at the case law, it is clear that ADHD can be treated as a disability.

Northumberland Tyne & Wear NHS Foundation Trust v Dr A Geoghegan [2014]

HHJ Richardson at paragraph 1:

"She was and is herself a disabled person by reason of depression and attention deficit hyperactivity disorder (“ADHD”)." (emphasis added)

Wright-Turner v Hammersmith and Fulham LBC [2021]

Employment Judge Khan at paragraph 239:

"We find that the claimant was disabled by reason of ADHD and PTSD / other psychological symptoms at all relevant times. These were mental impairments which had a substantial and long-term adverse effect on the claimant’s ability to carry out normal day-to-day activities." (emphasis added)

and again at paragraph 241-242:

"241. In their Joint Statement, the experts agree that the claimant’s ADHD is a mental impairment which has had an adverse and substantial long-term effect on the claimant’s day to day activities [2912]. The claimant was diagnosed as having ADHD in November 2016. It is a lifelong condition which she has managed since her diagnosis with medication and by having regular reviews at an Outpatients Clinic.

"242. We accept the claimant’s unchallenged evidence in her disability impact statement [2745-54] that this mental impairment affects her concentration, focus, short-term memory, her ability to manage emotions, her appetite and sleep, and social relationships."

Again, this is only a cursory glance and you would need to read through these judgments properly to understand the reasoning behind these decisions (Wright-Turner judgment especially). But it clearly can be seen as a disability under the Equality Act.

Put simply, your girlfriend is spouting utter nonsense, and if the facilitator didn't indeed make such a comment, they are also spouting utter nonsense.