I'm sorry she had a terrible experience. But the visa requirements are pretty clear, and she needed a standard visa.

Here's the "do you need a visa" website. https://www.gov.uk/check-uk-visa

Walk through it. She's from the USA. She's travelling for work, academic or business. She's planning to stay less than six months.

Here's the result:https://www.gov.uk/check-uk-visa/y/usa/work/six_months_or_le...

    You don't need a visa for some business and academic visits, but you must get a visa to work in the UK.
    You may be able to come to the UK without a visa if you:

    are invited as an expert in your profession
    come for other business or academic activities

[...]

    If you’re invited as an expert

    You can stay in the UK for up to 1 month without a visa, but you can only be paid to do certain things, eg:

    give guest lectures at a higher education institution
    provide advocacy in legal proceedings
    take part in arts, entertainment or sporting activities


    Check the full list of what you can be paid to do - it’s the same as what you can do on a Permitted Paid Engagement visa.

[...]

    If you come for other business or academic activities
    You can stay in the UK for up to 6 months without a visa, but you can only do certain academic or business-related activities, eg:

    go to a conference, meeting or training
    take part in a specific sports-related event
    perform as an artist, entertainer or musician
    do academic research or accompany students on a study abroad programme

    Check the full list of what you can do - it’s the same as what you can do on a Standard Visitor visa.

Here's the permitted paid engagement visa: https://www.gov.uk/permitted-paid-engagement-visa

Here's the standard visa: https://www.gov.uk/standard-visitor-visa