Wendy Chamberlain, the Liberal Democrat chief whip, said the decision not to progress with a criminal investigation into Patrick Grady does not excuse what the MP did.
In an interview with The Herald, Ms Chamberlain also described Ian Blackford as a “moral vacuum” and called on him to resign as the SNP’s Westminster leader.
Her comments come as SNP MP Patrick Grady is facing no further police action into allegations of sexual harassment.
The Glasgow North MP, who is now sitting as an independent, was suspended from parliament for two days after he was found to have made a sexual advance to a teenage SNP member of staff.
The Met launched its investigation last week after a complaint from a party staff member at Westminster.
The SNP previously said the Glasgow MP was “stepping away” from his membership while police carried out their inquiries.
However, it is not clear if Mr Grady will apply to rejoin the SNP and whether the party will accept his application.
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The sexual assault allegation was made on 22 June and submitted online by a third party.
It concerned an incident said to have taken place in October 2016 at a commercial premises in Folgate Street, London.
A Met Police spokesman said: “Officers carried out inquiries, including speaking to the alleged victim. There will be no further police action at this time.”
The Glasgow North MP was suspended from Parliament for two days after he was found to have “made an unwanted sexual advance” while “under the influence of alcohol” to an SNP staff member.
Mr Grady, the party’s former chief whip, told the Commons he was “profoundly sorry” for touching and stroking the neck, hair and back of the then 19-year-old colleague during a social event.
After Grady was suspended, it emerged Mr Blackford had urged his party’s MPs to “give as much support as possible” to the MP in an audio recording of an SNP group meeting which was leaked to the Daily Mail.
Nicola Sturgeon said she still had confidence in her party’s Westminster leader but said a victim who did not feel supported “is by definition unsupported”.
Lib Dem Ms Chamberlain said the SNP’s handling of the incident has left the party unable to comment on the allegations surrounding Tory MP Chris Pincher who faces various sexual assault allegations.
Ms Chamberlain -a former detective- said: “The SNP have been forced into allowing allegations of sexual misconduct within the Conservative party to pass without comment because they know they have failed at dealing with their own complaints.
“Just because the police have said they will not be conducting a criminal investigation into Patrick Grady does not mean that what he did was ok. Ian Blackford has been exposed as a moral vacuum, more concerned with protecting his friends than ensuring a young staff member got justice.
“Handling these types of allegations is something that all parties need to do better on. For the SNP, that should start with refusing to hand back the party whip to Patrick Grady and removing Ian Blackford from a position of leadership.”
The SNP has been contacted for comment.