Long response alert!
If you fail to proceed with extreme caution, you could make your friend's unsatisfactory situation infinitely worse and may ruin her chances of discovering the truth about her parentage.
It would seem that certain of her relatives put it into her head that the man named on her birth certificate may not be her biological father and this, together with her unfavourable treatment as a child of the family, has served to convince her that they may have been, or are, right.
Are any of the relatives who told her 'that ginger bastard isn't your father' alive? If so, does she have contact with them? If affirmative, she should approach them and ask outright what they meant by their statments. If she implies that she knows that 'ginger' isn't her papa, they may be forthcoming about her dm's relationships prior to her marriage and your friend's birth and it could be that names other than the late RDF are mentioned.
On the other hand, it could be these relatives told her untruths because they had some real or imagined score to settle with her dm and/or SDF. Or it could simply be that your friend has a markedly different physical appearance to her siblings - who may have auburn hair while hers is dark, for example - and this gave rise to idle speculation on their part.
The fact is that her dm may well be telling the truth and SDF's apparent aversion to her could be due to him feeling constrained to marry her mother because of her birth, which may or may not go some way to explain why they didn't marry prior to her arrival.
Is your friend able to be assertive with her immediate family members? She appears to have told her dm about her doubt, but has she raised this subject with SDF (acronym for sadistic damn fucker?)?
If I were your friend, SDF would be my second port of call and if he claimed he was my biological father I'd be asking him why he's treated me so badly/unfavourably and demand a DNA test to prove paternity and, if he wouldn't agree, I'd go through my siblings like a dose of salts until I found one that agreed to take a test to determine if we shared the same father.
From the way you've described the attitude of her sibllings, I would have have thought they'd be forming a disorderly queue to prove that she is 'less' than a child of the family than they are.
However, before any full frontal assault as it were, I would find opportunity to gather covert samples of DNA from SDF and a sibling or 2. A test carried out on a used tissue containing nasal fluid has an accuracy rate of 95% compared with 99% for mouth swabs and some 75% and below for items such as toothbrushes, cigarette butts etc, and only 25% for the famed licked stamp/envelope.
Tests on hair strands are 99% accurate but require 5-10 fresh hairs with roots attached for comparison which may rule out combs/hairbrushes unless they are only used by the subject and happen to contain fresh hairs with their roots.
Please note that it may be necessary to send covertly obtained DNA samples outside of the UK for testing and, if it is proved that SDF is not your friend's father, she's best advised not to advertise how she acquired her knowledge but to assert it until such time as he/her siblings agree to conventional DNA testing which can be carried out by a UK lab.
Only after every effort has been made to extract (no pun intended) the truth by overt or covert means should any thought be given to making an approach to any of the late RDF's family members.
If it is deemed necessary to make an approach to what may possibly be your friend's biological df's family, I would suggest that it should be made to one of his siblings who should be best placed to know whether their brother enjoyed a relationship with her dm around the time your friend was conceived.
Alternatively, if she is cordial terms with any of RDF's children, she could perhaps mention obliquely in passing that she's always wondered who her df was and note their reaction before continuing the conversation, either at that time or raisng it again at a later date.
I appreciate you want to end your friend's uncertainty as quickly possible but I caution you once more against acting in haste. The fact is that your friend has lived with this doubt for many years and another few weeks or months, while knowing that she may achieve some resolution that wasn't in prospect for her until you came on the scene, isn't going to add to her existing hurt.
Feel free to pm me if you should need any further info on covert DNA testing and laboratories with the means to make the necessary comparisons, or any aspect of your friend's quest.