Thank you for your query.
From what I understand from your question, you a looking to bring your friend to the UK on a tourist visa. Firstly, your friend as the applicant has to show that she satisfies the requirement of the visit visa rules which is set out at:
Your friend must show that she has every intention to return to Thailand. From what you have stated, it is clear that she has every intention to return to Thailand as she has employment and a son. She will still have to show that she can fund her travel and maintenance and has accommodation in the UK.
As a visitor, she does not need a TB certificate because she will be coming to the UK for less than six months under the standard tourist visa.
Finally, I do not understand your point about showing that you are in a genuine relationship. If you are looking to bring your partner permanently in the UK, then you need to satisfy Appendix FM of the rules. https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-fm-family-members
Under Appendix FM, the Embassy considers the genuine relationship as someone who has been together for two years or they are married. The other option, which I do not know you may be interested or not is Marriage Visitor route: https://www.gov.uk/marriage-visa/overview
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